Add support for multi-schema migrations in Postgres

[vporoshok]

I've save interface of function GenerateAdvisoryLockId with variadic params. So I've just add test to ensure that lock id depend on this additional parameters.

[coveralls]

Pull Request Test Coverage Report for Build 215

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 46.883%

---

Totals
Change from base Build 210:	0.07%
Covered Lines:	707
Relevant Lines:	1508

---

💛 - Coveralls

[dhui]

Thanks for the PR!

[dhui]

Use strings.Join() instead of bytes.NewBufferString() since it's more readable (less code) and faster in all cases (except when there's one string)

[dhui:benchmark_buf_join]$ cat bench_test.go 
package test

import (
	"bytes"
	"strings"
	"testing"
)

const base = "base"

var strs = [][]string{
	{},
	{"foo"},
	{"foo", "bar"},
	{"foo", "bar", "hello", "world"},
}

func Buf(s string, others ...string) []byte {
	buf := bytes.NewBufferString(s)
	for _, s := range others {
		buf.WriteByte(0)
		buf.WriteString(s)
	}
	return buf.Bytes()
}

func Join(s string, others ...string) []byte {
	others = append(others, s)
	return []byte(strings.Join(others, "."))
}

func BenchmarkBuf(b *testing.B) {
	for _, s := range strs {
		b.Run("", func(b *testing.B) {
			for i := 0; i < b.N; i++ {
				Buf(base, s...)
			}
		})
	}
}

func BenchmarkJoin4(b *testing.B) {
	for _, s := range strs {
		b.Run("", func(b *testing.B) {
			for i := 0; i < b.N; i++ {
				Join(base, s...)
			}
		})
	}
}
[dhui:benchmark_buf_join]$ go test -bench . -benchmem
goos: darwin
goarch: amd64
BenchmarkBuf/#00-8 	20000000	        79.2 ns/op	     120 B/op	       2 allocs/op
BenchmarkBuf/#01-8 	20000000	        92.8 ns/op	     120 B/op	       2 allocs/op
BenchmarkBuf/#02-8 	10000000	       210 ns/op	     152 B/op	       3 allocs/op
BenchmarkBuf/#03-8 	 5000000	       335 ns/op	     200 B/op	       4 allocs/op
BenchmarkJoin4/#00-8         	20000000	        70.4 ns/op	      24 B/op	       2 allocs/op
BenchmarkJoin4/#01-8         	10000000	       141 ns/op	      48 B/op	       3 allocs/op
BenchmarkJoin4/#02-8         	10000000	       177 ns/op	      96 B/op	       3 allocs/op
BenchmarkJoin4/#03-8         	10000000	       228 ns/op	     224 B/op	       4 allocs/op
PASS
ok  	benchmark_buf_join	15.506s
[dhui:benchmark_buf_join]$ 

[vporoshok]

Wow! It's a good tip! Thank you!

[dhui]

Use []string and add a test case for multiple strings

[dhui]

I like the usage of variadic args!

[vporoshok]

Thank you!

[dhui]

I'm not a fan of the schema name being exported, but you're just following the existing pattern where the database name is exported. Let's leave this as is for now and we'll tackle unnecessarily exported field names in a later backwards incompatible version of migrate.

[vporoshok]

Thank you!

[dhui]

Thanks for reporting and fixing the issue!